This invention relates to power control for RF signals, and particularly to power control of RF signals in a mobile communication network.
In a mobile communication network, a base transceiver system transmits and receives radio signals between a base station and mobile stations. It is necessary to control the transmitted power levels of the radio signals and this is done in dependence on a variety of parameters in accordance with the GSM and other standards. In any mobile communication system, it is necessary to have fairly accurate control over the Dower levels of the transmitted signals in order to ensure that they reach mobile stations with an adequate but not excessive power level, and to prevent interference between neighbouring base stations operating in the same frequency channel.
Cell configurations vary widely. For large cells, or for cells where there is considerable signal attenuation, it is desirable to boost the transmitted power level. This can be done using a booster amplifier which amplifies the already amplified signal. A booster may not always be necessary and therefore it is particularly desirable to provide a system in which a booster can be included if necessary with minimal modifications to the remainder of the system. In particular, it is desirable to avoid the use of over-complex circuitry and the need for user setup each time a booster is introduced.
This problem has not previously been satisfactorily solved since in existing systems all the necessary power amplification has been included within a single mechanical confinement. Moreover, in order to accurately control the power levels, a power levelling loop is utilised. It has previously been assumed that the incorporation of the booster into the amplification circuitry and within the power levelling loop would require multiple analogue connections and complex hardware and software modifications to existing products. Moreover, with a boosted power level the problem of noise pick-up becomes more significant.
According to one aspect of the present invention there is provided a power levelling loop for a base transceiver system comprising:
first amplification circuitry connected to receive an RF input signal and to supply a first amplified signal to an output node;
first detection circuitry for detecting said first amplified signal and connectable to modify the amplification factor of the first amplification circuitry in dependence on the detected level of the first amplified signal;
switch circuitry having a first input connected to the first detection circuitry and a second input connected to means for receiving a detected level of a second amplified signal, the switch being controllable to select the first or second input, wherein when the second input is selected the amplification factor of the first amplification circuitry is modified in accordance with the detected level of the second amplified signal.
In the preferred embodiment, a coaxial cable is connected to the output node. The preferred embodiment of the invention centres on the use of a closed loop system utilising one cable for two paths. The cable can be used to transmit the first amplified signal (the amplified RF signal) from the first amplification circuitry and to feed back the detected level of the second amplified signal (the boosted signal) to the power levelling loop. The detected level of the boosted signal is an analogue voltage which varies at a very low frequency compared to the RF input signal. Thus it is possible to use the coaxial cable as a path both for the detected level of the boosted signal and for outputting the amplified RF signal.
In a modular base transceiver system (BTS), the power levelling loop can be included in a transceiver module which has an input node for receiving the RF input signal and which provides the output node from which the first amplified signal is supplied. Such a modular system can also include a booster module for a base transceiver system comprising:
an input node for receiving a first amplified signal
a booster amplifier for generating a boosted signal from the amplified signal;
a detection circuit for detecting the boosted signal and for generating a detected level of the boosted signal; and
means for transmitting the detected level of the boosted signal to a power levelling loop via the input node of the booster module.
The transmitting and receiving means can each comprise a device for allowing passage of the low frequency detected level of the boosted signal while preventing passage of the high frequency RF signal. This can be implemented by decoupling the transmitting means and receiving means from the high frequency RF signals, for example by use of an inductance or an RF choke.
In the preferred embodiment, as only existing cabling is used between the output of the transceiver module and the input of the booster module, changes to other modules within the BTS are minimised. Moreover, the possibility of noise pick-up is minimised since the feedback path is maintained in a coaxial cable and does not pass near any high speed digital signal lines.
It will be noted however that while the use of a common coaxial cable represents a particularly useful and advantageous implementation of the invention, it would also be possible to implement transceiver and booster modules where the detected level of the second amplified signal is fed back to the power levelling loop in some other manner. For example, an extra jumper accessible on the backplane of a rack for receiving the modules could be used. This would require a carefully designed backplane to avoid the problems of noise pick-up.
For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings.